Process of making homogeneous particles comprising nitrocellulose mixtures



Feb. 15, 1966 c. v. MURPHY 3,235,420 PROCESS OF MAKING HOMOGENEOUS PARTICLES COMPRISING NITRQCELLULOSE MIXTURES Flled May 4, 1962 AQUEOUS SLURRY OF EMULSIFIERS AND SOLVENT ADDITION OF EXPLOSIVE MIXTURE REFLUX CENTRIFUGE DRYING INVENTOR CHARLES V. MURPHY r BY ATTORNEY United States Patent PROCESS OF MAKING HOMOGENEOUS PAR- TICLES COMPRISING NITROCELLULOSE MIXTURES Charles V. Murphy, Merchantville, N.J., assignor to E. I. du Pont de Nemours and Company, Wilmington, Del., a corporation of Delaware Filed May 4, 1962, Ser. No. 192,429 12 Claims. (Cl. 149-18) The present invention relates to explosive composi tions in the form of small, dense spheres and to a process for their manufacture.

Solid propellant compositions are of considerable interest in the field of rocketry, missiles and the like. In forming solid propellant grains it is necessary to include some material to behave as a binder for the more energetic compounds used. These energetic materials include ammonium perchlorate, ammonium nitrate, etc. Binders which have been used include urethane polymers and the like. Binders of this type, however, dilute the energy content of the propellant, therefore binders containing energetic groups are desired. Nitrocellulose in the form of small spheres has been used as a binder because such spheres have the property of being incorporated into propellant mixes containing plasticizers easily and thoroughly to give stable fluid dispersions followed by a slow and controllable gelation as the particles of nitrocellulose solvate. Compositions of nitrocellulose and higher energy materials such as cyclotetramethylenetetranitramine (HMX) and cyclotrimethylenetrinitramine (RDX) have a higher heat of decomposition than nitrocellulose alone and thus have more energy.

The present invention provides small, dense, spherical, homogeneous particles of nitrocellulose admixed with a solid, organic, water-insoluble, non-polymeric explosive.

These small, dense spheres are produced by a process comprising mixing (a) nitrocellulose, (b) a solid, organic, water-insoluble, non-polymeric explosive, (c) water, (d) an organic solvent both for nitrocellulose and for non-polymeric explosive, (e) a non-ionic emulsifier, and (f) an anionic emulsifier, distilling off the organic solvent and separating said spheres from the aqueous residue.

The nitrocellulose used in making the products of the present invention should have as high a nitrogen content "as possible without impairing its solubility properties.

While the nitrocellulose does not go into solution during the process of preparing the dense spheres of the present invention, the solubility properties of the nitrocellulose 'play an important role in casting the final propellant grain because of their influence on the gelation time in forming the grain and the strength and elongation properties of the finished propellant. The nitrogen content of the nitrocellulose usable in the present invention varies from about 12% to about 13%. The grade of nitrocellulose, known in the art as pyro nitrocellulose, and having a nitrogen content of about 12.6% is the preferred grade for practicing this invention.

The solid, organic, water-insoluble, non-polymeric explosive used in conjunction with the nitrocellulose will be one having a high-energy content. Examples are pentaerythritol tetranitrate (PETN), HMX, RDX and many others. The preferred explosives are HMX and RDX.

The ratio of nitrocellulose to non-polymeric explosive can vary from about 40/60 to about /90. Nitrocellulose serves as a binder for the non-polymeric explosive so that too little of it gives a fragile particle. Too much nitrocellulose is not disadvantageous except from the standpoint of low relative energy content, but generally as little nitrocellulose as possible will be used.

The solvent for nitrocellulose and the non-polymeric explosive must not be miscible With water and preferably less than about 5% soluble in water. It must boil below about 100 C. or form an azeotrope with water boiling at less than about 100 C. The preferred solvents for practicing this invention are isopropyl acetate and butyl acetate.

In order to obtain the eminently satisfactory products of this invention, emulsifying agents are included in the reaction medium. Without them the particles do not have the spherical shape and controlled size. There must be at least one anionic emulsifying agent admixed with at least one non-ionic emulsifying agent. Anionic emulsifying agents suitable in the practice of this inven-.

tion are sodium dialkylsulfosuccinates such as sodium di(2-ethylhexyl)sulfosuccinate. Non-ionic surface active agents suitable in the practice of this invention are polyoxyethylene sorbitan monolaurate (sold as Tween 20), polyoxyethylene sorbitan monopalmitate (sold as Tween 40), polyoxyethylene sorbitan monooleate (sold as Tween 80).

The total emulsifier content must be at least 2% based on the weight of the water present. The upper limit is based only on economics but preferably would not be above 6%.

The ratio of non-ionic emulsifier to anionic emulsifier can range between about :30 to about 40:60. The preferred range lies between about 65:35 and 55:45.

The .particle size of the non-polymeric explosive/nitrocellulose spheres is influenced by the water:(explosive +nitrocellulose) and the solvent:(explosive-l-nitrocellulose) ratios. This average particle size ranges from about 25 microns to about 200 microns. Although I do not wish to be bound by theory I believe that the greater the quantity of liquid (water+solvent) the more finely subdivided the particulate material can be with a given degree of agitation. The s0lvent:(nitrocellulose+explosive) ratio can vary from about 6/1 to about 4/ 1. The water:(nitrocellulose+explosive) ratio can vary from about 7/1 to about 5/1.

During the period of formation of the spherical particles the contents of the reaction vessel are first mixed at room temperature and then heated to the reflux temperature of the solvent-water azeotrope. The mixing at room temperature serves to uniformly distribute the ingredients before heating to distill off the solvent. The length of this mixing can vary from about 20 to about 60 minutes.

After all of the solvent-Water azeotrope is distilled off the heating is continued until the vapor temperature reaches that of the boiling water. This final heating serves merely to remove the last traces of organic solvent.

Good agitation is required for the instant process in order to properly disperse the heterogeneous phases present and to get spherical particles. The speed of agitation needed depends on the construction of the vessel and agitator blade and on the size of the vessel and agitator blade. A vessel having baflles will require a lower speed of agitation than one without baflles, for example. In general, turbulent agitation is needed.

Anti-foaming agents may optionally be added to the reaction mixture during distillation to prevent carrying over of entrapped liquid during the distillation of the solvent-water azeotrope. These antifoaming agents are not essential to the invention but merely reduce the time necessary for distilling off the solvent. Silicone oils are suitable anti-foaming agents.

Conventional nitrocellulose stabilizers can be included in the formulation but play no part in obtaining the unique physical form of the products of the instant invention. They serve merely to prevent autocatalysis of the decomposition of the nitrate ester by absorbing nitrogen oxide fumes as they are formed on standing over long periods. Such stabilizers as diphenylamine, 2-nitro-diphenylamine, and the centralites are useful.

The propellant particles of the instant invention are characterized by being essentially spherical, dense and non-porous. At least 60% of the particles in any sample of the propellant are spherical with the remaining particles being elongated spheres and rods. The essentially spherical nature of the particles gives a product having a higher bulk density than that of a product having non-spherical particles, which is easily incorporated into a plasticizer. The particle density of the propellant ranges from about 1.6 to 1.8 g./ml. The spheres of the instant product vary in average diameter from 25 microns to 200 microns with essentially no particles having a diameter greater than 250 microns. These particles increase the gelation time when they are placed in a plasticizer or solvent therefor over that obtained with more porous particles. This increased gelation time is advantageous because it allows more time for compounding a finished propellant before the composition sets up.

The nitrocellulose-HMX composition of the present invention is surprisingly less sensitive to ignition by static than nitrocellulose alone in the same spherical form. The static sensitivity is measured in the following way. The sample (0.05 g.) is placed in a depression in a brass plate and an electrical spark of known energy is discharged through the material from a needle electrode 5 mm. above the plate. The spark energy is reduced in 0.05 joule increments until ignition no longer occurs. This point is called the static sensitivity.

The nitrocellulose-HMX composition described in Example 1 has a static sensitivity of 1.93 joules Whereas nitrocellulose alone in the same spherical form has a static sensitivity of 1.1 joules. This means that the nitrocellulose is ignited by a spark having less energy than that necessary to ignite the nitrocellulose-HMX mixture.

The process of the instant invention is more readily understood by reference to the drawing which, schematically depicts the general process step requirements. The aqueous slurry is formed in a reactor and comprises water, at least one anionic and at least one non-ionic emulsifier and a solvent for nitrocellulose and a solid, organic, water-insoluble, non-polymeric explosive, preferably RDX or HMX. The slurry is agitated and the nitrocellulose and the non-polymeric organic Water-soluble explosive, i.e. HMX or RDX are added while agitation continues. The reactor is then heated to reflux and the water and the solvent are distilled off, whereupon the reaction mixture is fed into a preheated centrifuge to separate the liquids from the solids. After separation the solid explosive spheres of nitrocellulose and non-polymeric explosive, i.e., preferably RDX and HMX, are recovered.

The invention is illustrated by the following examples. Parts, where given, are by weight.

Example 1 Polyoxyethylene sorbitan monolaurate (Tween 20) (220 parts) and sodium di(2-ethylhexyl)sulfosuccinate (Aerosol OT) (140 parts) are dissolved in 200 parts of isopropyl acetate. This solution is added to 12,000 parts of Water in a steel tank fitted with a heating jacket and a paddle-blade agitator and mixed briefly. Isopropyl acetate (9800 parts) is added and mixed well. HMX (1420 parts), pyro nitrocellulose (580 parts) and 2-nitrodiphenylamine (30 parts) are added and mixed for about 30 minutes with an agitator speed of 300 r.p.m. The reactor is then heated to reflux and the isopropyl acetatewater azeotrope is distilled off at about 76 C. while continuing agitation. After the azeotrope has been distilled off, the vapor temperature is allowed to reach 100 C. The hot reaction mixture is then fed into a preheated cen- 4 trifuge and the liquid separated from the solid. The solid spheres of HMX-nitrocellulose are dried at 55 C. These spheres have an average diameter of 150 microns and a particle density of 1.76. Eighty-five percent of the particles are spheres, and the remaining particles are elongated spheres and rods.

Example 2 The exact procedure of Example 1 is repeated using the following ingredients:

Parts Water 10,000 Tween 20 150 Aerosol OT 150 Isopropyl acetate 8,000 RDX 1,420 Pyro nitrocellulose 580 2-nitrodiphenylamine 30 The product is again in the form of dense spheres having an average particle diameter of 125 microns. Eighty percent of the particles are spheres and the remainder elongated spheres and rods.

When the above example is repeated using 8000 parts of butyl acetate instead of isopropyl acetate and distilling olf the butyl acetate-water azeotrope at 90.5 C., the product is essentially identical with the one above.

Example 3 Polyoxyethylene sorbitan monolaurate (1.8 parts) and sodium di(2-ethylhexyl)sulfosuccinate (1.2 parts) is dissolved in 20 parts of isopropyl acetate. This solution is added to a mixture parts of water and 100 parts of isopropyl acetate and mixed well. HMX (7 parts) and pyro nitrocellulose (3 parts) are added and the mixture agitated for about 30 minutes. The mixture is heated to reflux and the azeotrope of isopropyl acetate and water is distilled off at about 76 C. while continuing agitation. After the azeotrope is distilled off, the vapor temperature is allowed to rise to 100 C. The hot slurry is then centrifuged to separate the liquid from the solid. The solid consists of essentially spherical particles having an average particle diameter of 32 microns. Ninety-five percent of the particles are smaller than 62 microns in diameter. Sixty percent of the particles are spheres, 30 percent are elongated spheres and 10 percent are rods.

The invention has been completely described above. Many other modifications will be apparent to those skilled in the art without departing from the inventive concept.

I claim:

1. A process for making explosive compositions in the form of :small, dense, essentially spherical, homogeneous particles, which comprises forming an aqueous dispersion of a mixture of (1) nitrocellulose, (2) a member selected from the group consisting of pentaerythritol tetranitrate, cyclotetramethylenetetranitramine and cyclotrimethylene trinitramine, (3) an organic solvent for nitrocellulose, said solvent being less than about 5% soluble in water, and being selected from the group consisting of nitrocellulose solvents boiling below about 100 C. and those forming azeotropes with water boiling below about 100 C., (4) an anionic emulsifier selected from the group consisting of sodium dialkylsulfosuccinates, and (5) a non-ionic emulsifier selected from the group consisting of polyoxyethylene sorbitan monolaurate, polyoxyethylene sorbitan monopalmitate and polyoxyethylene sorbitan monooleate, the total emulsifier content being at least 2% by weight of the Water present in said dispersion, heating said dispersion at a temperature sufficient to distill off the organic solvent, separating the liquid and solid phases and recovering the solid particles.

2. The process of claim 1 wherein the solid non-polymeric explosive is cyclotetr-amethylenetetranitramine.

3. The process of claim 2 wherein the anionic emulsifier is sodium di(2-ethylhexyl)sulfosuccinate and the nonionic emulsifier is polyoxyethylene sorbitan monolaurate.

4. The process of claim '1 wherein the solid non-polymeric explosive is cyclotrimethylenetrinitramine.

5. The process of claim 1 wherein said solvent is isopropyl acetate.

6. The process of claim 1 wherein said solvent is butyl acetate.

7. A process for the manufacture of small, dense, essentially spherical, homogeneous particles comprised of nitrocellulose and an explosive selected from the group consisting of pentaerythritol tetranitrate, cyclotetramethylenetetranitramine, and cyclotrimethylenetrinitramine which comprises: (a) dissolving a mixture of an anionic emulsifying agent selected from the group consisting of sodium dialkylsulfosuccinates and a non-ionic emulsifying agent selected from the group consisting of polyoxyethylene sorbitan monolaurate, polyoxyethylene sorbitan monopalrnitate and polyoxyethylene sorbitan monooleate in an organic solvent for nitrocellulose which is less than about 5% soluble in water and is selected from the group consisting of nitrocellulose solvents boiling below about 100 C. and those forming azeotropes with water boiling below about 100 C., (b) mixing the above solution with water and an additional quantity of said organic solvent, the total emulsifier content in the mixture thus prepared being at least 2% by weight of the water present, (c) adding said explosive and nitrocellulose in a ratio ranging from about 60/40 to about 90/10 to the mixture from step (b) and mixing well, said nitrocellulose having a nitrogen content of from about 12% to about 13%, (d) distilling off said organic solvent while maintaining agitation, (e) separating said spherical particles from the water present.

'8. The process of claim 7 wherein the non-ionic emulsifying agent and the anionic emulsifying agent are present in a ratio by weight varying from about 70/30 to about /60.

9, The process of claim 7 wherein the non-polymeric explosive i-s cyclotetramethylenetetranitramine.

10. The process of claim 7 wherein the non-polymeric explosive is cyclotrimethylenetrinitramine.

1'1. The process of claim 7 wherein said solvent is isopropyl acetate.

12. The process of claim 7 wherein said solvent is butyl acetate.

References Cited by the Examiner UNITED STATES PATENTS 2,027,114 1/1936 Olsen et a1. 1492 2,425,854 8/1947 Alexander 149-92 2,916,996 12/1959 Coffee 14992 X CARL D. QUARFORTH, Primary Examiner. LEON D. ROSDOL, Examiner. 

1. A PROCESS FOR MAKING EXPLOSIVE COMPOSITIONS IN THE FORM OF SMALL, DENS, ESSENTAILLY SPHERICAL, HOMOGENEOUS PARTICLES, WHICH COMPRISES FORMING AN AQUEOUS DISPERSION OF A MIXTURE OF (1) NITROCELLULOSE, (2) A MEMBER SELECTED FROM THE GROUP CONSISTING OF PENTAERYTHRITOL TETRANITRATE, CYCLOTETRAMETHYLENETETRANITRAMINE AND CYCLOTRIMETHYLENETRINITRAMINE, (3) AN ORGANIC SOLVENT FOR NITROCELLULOSE, SAID SOLVENT BEING LESS THAN ABOUT 5% SOLUBLE IN WATER, AND BEING SELECTED FROM THE GROUP CONSISTING OFNITROCELLULOSE SOLVENTS BOILING BELOW AOUT 100*C. AND THOSE FORMING AZEOTROPES WITH WATER BOILING BELOW ABOUT 100* C., (4) AN ANIONIC EMULSIFIER SELECTED FROM THE GROUP CONSISTING OF SODIUM DIALKYLSULFOSUCCINATES, AND (5) A NON-IONIC EMULSIFIER SELECTED FROM THE GROUP CONSISTING OF POLYOXYETHYLENE SORBITAN MONOLAURATE, POLYOXYETHYLENE SORBITAN MONOPALMITATE AND POLYOXYETHYLENE SORBITAN MONOOLEATE, THE TOTAL EMULSIFIER CONTENT BEING AT LEAST 2% BY WEIGHT OF THE WATER PRESENT IN SAID DISPERSION, HEATING SAID DISPERSION AT A TEMPERATURE SUFFICIENT TO DISTILL OFF THE ORGANIC SOLVENT, SEPARATING THE LIQUID AND SOLID PHASES AND RECOVERING THE SOLID PARTICLES. 